Commonly owned international application No. PCT/AU93/00649 (WO 94/13946) discloses an engine control unit (ECU) in which each of the following engine operating parameters are detected: engine speed, throttle position, manifold absolute pressure (MAP), gas pressure, gas temperature, battery voltage, manifold air temperature, engine phase and boost pressure control valve (BPCV) feedback position. Each of these parameters is provided as an input in the form of an electrical signal to the ECU controller, for controlling the operation of a gas fuelled internal combustion engine. The ECU controller typically employs the detected engine speed and throttle position to calculate a percentage full load (PFL) value for the engine. PFL is a non-dimensional measure of the load or torque produced by or required from the engine. The calculated PFL value can then be employed by the ECU controller to calculate the injector ON time (IOT) for each gas injector in a gas delivery system for the engine. Typically, the PFL value is employed together with the engine speed to calculate a required manifold absolute pressure (MAP) value. The calculated value of required MAP may then be employed, together with the PFL value, to calculate a percent allowable load (PAL) value. The PAL value is then employed by the ECU controller to calculate the IOT and spark advance for the engine. By employing gas injectors and accurately calculating the IOT, the ECU can control the correct amount of gaseous fuel to be injected into each cylinder to achieve optimum engine performance under the full range of engine speed and load conditions. The disclosure of WO 94/13946 is incorporated herein by reference.
The ECU of WO 94/13946 operates very satisfactorily and is still in use in some gas fuel vehicles. However, certain improvements to the ECU were perceived as desirable to provide better control over the delivery of air and gas to each cylinder of the engine. Compliance with the stringent exhaust emission standards now being introduced in many countries, which aim to reduce the level of NO.sub.x gases, carbon monoxide and hydrocarbons in engine exhaust fumes was also a factor. It was thought desirable to be able to provide a greater degree of control over the air:fuel ratio in each cylinder of the engine so that the engine can operate as lean as possible (to reduce NO.sub.x) without causing engine misfire to occur.